Semiconductor manufacturers must go to great lengths to maintain a clean processing environment and to prevent particle contamination of critical wafer surfaces. Sources of such particle contamination are myriad. In a relatively still air environment particles may emanate from moving machinery such as electric motors or robotic devices. In an environment where the air is rapidly moving, surface particles may literally be emitted by most anything. Once the particle is airborne, it becomes a potential contaminant whether it comes from a moving machine or from a surface. In either case it is prudent to eliminate or lessen the source of particles. Where the source cannot be eliminated, it is appropriate to reduce the deposition of airborne particles on surfaces. One way to do so is with bi-polar air-ionization.
A number of studies have identified electrostatic forces as having a major influence on the rate of particle deposition onto charged surfaces. In many instances electrostatic forces can be more significant than gravitational, aerodynamic, or thermal forces. Bipolar air ionization can effectively reduce surface charges, and as a result decrease the rate of particle deposition on critical product surfaces. Several tests have shown a significant improvement in surface particle counts on test wafers when the surface potential was reduced from 5 kV to &lt;100 volts.
However, other reports indicate that some high voltage DC corona discharge ionizers themselves generate large numbers of submicron particles in the 0.003 to 0.03 micron range. The small size of these particles would allow the apparent contradiction with reports of lower surface contamination in ionized areas, since these small particles would remain undetectable by most surface scanning equipment.
The papers reporting particle generation have drawn criticism in several respects. Some results were based on accelerated testing at corona currents up to 50 times normal operating levels. Other tests failed to report such factors as the placement and grounding of the particle sampling probe, the air flow past the ionizer and probe, and the background particle counts. Some of the tests used emitter materials that ionizer manufacturers do not use because these materials erode rapidly. Particle emission from moving machinery such as robotics or electric motors, while known or suspected, is apparently not well reported.
Overall, the lack of standardization in the test methods prevented any comparison of the particle readings from different materials and articles including ionization devices. Finally, disagreement exists as to the source of the particles. For example, an analysis of the deposits formed on ion emitters in cleanrooms shows that the residue is nonmetallic and probably collected from some other source in the environment.
Ultrafine particle generation concerns manufacturers of equipment used in a cleanroom and their customers. Clearly a standardized test chamber and method is needed for measuring particle emissions from any equipment that is to be used in the manufacture of electronic equipment.
In the past a number of methods have been used to obtain measurement of particle emission from semiconductor processing equipment. These methods include: